1. Field of the Invention
The invention concerns X-ray scanners and, more particularly, a method for the calibration of machines of this type using one or more circular phantoms which are off-centered with respect to the axis of rotation of the scanner. The invention also concerns a system which can be used to implement this method.
2. Description of the Prior Art
To examine patients, increasingly frequent use is being made of X-ray machines, called scanners, which make cross-sectional images of these patients. These machines are based on the physical phenomenon of X-ray absorption by the human body. For a monochromatic beam, this absorption is directly related to the distance x travelled by the X-rays in a homogeneous body according to the formula: EQU I=I.sub.o e-bx
a formula wherein:
I.sub.o is the intensity of the radiation entering the human body, PA1 I is the intensity of the radiation leaving the human body, PA1 b is a coefficient of X-ray absorption depending on the body through which the radiation flows. PA1 positioning said circular phantom, the center of which is placed in a position that is off-centered with respect to the axis of rotation of the scanner, PA1 rotating the scanner to make m attenuation measurements Y.sub.ij for each order i channel, each measurement corresponding to an angular position .beta..sub.j of the scanner, PA1 means for placing at least one circular phantom between the X-ray source and the detection device in a position that is off-centered with respect to the axis of rotation of the rotating structure, PA1 means for rotating the scanner to m different angular positions .beta..sub.j close to one another, PA1 means for computing the attenuations Y.sub.ij, undergone by the X-ray radiation in the phantom, from the signals received on the N detectors, PA1 means for recording the N values of Y.sub.ij for each of the m angular positions .beta.j, PA1 means for computing from the N.m values of Y.sub.ij, the off-centered coordinates of the phantom with respect to the axis of rotation of the scanner, PA1 means for computing, from the off-centered coordinates, the lengths X.sub.ij of the paths of the X-ray radiation in the phantoms as well as the corresponding attenuations b.X.sub.ij. PA1 means for comparing the values of Y.sub.ij with the computed values of the attenuations b.X.sub.ij for the same channel and for computing the coefficients A.sub.ik of the polynomial approximation and, PA1 means for recording said coefficients A.sub.ik thus computed, so as to subsequently use them as multiplying coefficients of the attenuations Y.sub.ij undergone by the X-radiation in the body of the patient.
In a logarithmic measurement scale, the attenuation I/Io is equal to bx, i.e. it is proportional to the distance x.
As shown in FIG. 1, these machines consist essentially of an X-ray source 10 which is associated with a detection device 11, said two elements being arranged with respect to each other in a fixed geometrical relationship so that the body to be examined can be interposed between them. Furthermore, they are supported by a structure (not shown) which can rotate around the body to be examined so as to irradiate the body at different angles. The X-ray source, which is controlled by a device 13, radiates within an angular sector with a width sufficient to illuminate the entire cross section of the body. The detection device 11 has the shape of an annular sector, the length of which is suited to the width of the X-ray beam and consists of a large number of elementary detectors 12, placed beside one another.
To obtain an image of the cross section of the human body through which the X-ray beam flows, the supporting structure of the source 10 and the detection device 11 is made to rotate around the body and the output signals of the elementary detectors 12 are measured so as to be processed appropriately according to known methods in order to extract therefrom an image representing the cross section. For this processing operation, the elementary detectors 12, which are also called channels, are connected to an electronic device 14 which first does a computation of the logarithm of the received signals so as to obtain a signal with an amplitude which is proportional to the attenuation of the X-rays.
Owing to the effect of different phenomena, which shall not be explained herein, the amplitude of this signal for each elementary detector or channel is not proportional to the attenuation or absorption that is actually undergone. Consequently, to cope with this drawback, several methods have been thought of. These methods consist, for example, in sampling the output signals of the channels in the presence of bodies called phantoms the dimensions and absorption coefficients of which are known in such a way as to compute (by logarithmic computations) the attenuations and to compare these measured attenuations with values computed as a function of the dimensions and the absorption coefficients of the phantoms. These comparisons enable the computation of a relationship between the measured values and the values that should have been obtained. This relationship may take the form of files or mathematical formulae for each detection channel.
The phantoms used to perform these so-called calibration measurements are, for example, phantoms of different thicknesses which are introduced in the vicinity of the X-ray source, which implies handling operations at the source to insert and withdraw these phantoms.
The U.S. Pat. No. 4,352,020 proposes the use of circular phantoms 15, 16 and 17, of different diameters, placed at the center of rotation of the supporting structure. This embodiment is close to the conditions of the measurements which will be performed on the body to be examined. This patent also proposes the use of a cone-shaped phantom with a circular section which is moved crosswise with respect to the beam so as to obtain different absorption lengths. With the phantoms described, the measurements are made for a determined position of the supporting structure and for each phantom.
FIG. 2 shows the shape of three response curves 20, 21 and 22 of the attenuation as a function of the position of the channels in the case of measurements on three circular shaped phantoms. The measured values are shown by dots and vary around a mean value which represents the theoretical value in a linear system. These curves can be used as follows: when the measured signal corresponds to a point A, it will be deduced therefore that the linear signal is the point A' of the mean curve 20. When the measured signal corresponds to a point B located between the curves 20 and 21, the linear signal will be deduced therefrom by interpolation between the curves 20 and 21. This interpolation can be computed according to a linear relationship or, more generally, according to a polynomial relationship.
The curves 23 and 24 of FIG. 3 show the principle of the calibration of a channel in another way. These are curves that describe, in a given channel, the attenuation as a function of the thickness x for measured values (curve 23) and for computed values (plain line 24): in fact, the measured values give points which are connected to one another according to a given relationship, which may be linear or polynomial, so as to obtain a continuous curve. When an attenuation is measured, it corresponds, for example, to the point C of the curve 23 and the linear value corresponding to the point C' of the curve 24 is deduced therefrom.
The above-mentioned U.S. patent describes an instrument in which the correspondence between the measured and real attenuation values is obtained by a system of files created during the calibrating operation. With respect to the interpolation, the patent proposes linear, cubic or bi-quadratic interpolations but only the linear interpolations are described in detail.
The calibrating methods that have been briefly described above have the major drawback of using several phantoms, and this entails numerous handling operations. Furthermore, these handling operations have to be precise, especially for circular phantoms different centers of which should coincide with the center of rotation of the structure.
It must be noted that the above cited US patent proposes the use of a single phantom which would have the shape shown in FIG. 12 of said patent and proposes making the structure rotate around this phantom: this results in obtaining absorption paths of different lengths depending on the angular position of the structure. However, such an operative procedure is only cited and there is no indication concerning the method or the means for the implementation thereof.
In brief, the purpose of the calibrating methods is to homogenize the response of the channels to one another and to move towards a linear response. To obtain this linear response, a polynomial correction is applied to each channel and this correction is computed, i.e. the coefficients of the polynomial are determined, from measurements made from different values of the absorption path. As such to determine the coefficients of an n order polynomial of a determined channel, it is necessary and sufficient to make (n+1) measurements of different paths for this channel.
In the prior art examples described above, it has been shown that, for example, (n+1) phantoms of different thicknesses or (n+1) circular phantoms of different diameters are used, the values of the thicknesses and diameters being chosen to cover the required dynamic range. This therefore results in numerous handling operations to change from one phantom to another, and these handling operations have to be precise for circular phantoms, the centers of which should coincide with the center of the rotating structure.